Plans to fortify flour with a vitamin to stop birth defects should be delayed until there has been more research into possible ill-effects, say scientists.

Writing in the British Medical Journal, two child health experts question whether increasing the folic acid intake of 50m people is the right move in the absence of any firm evidence of safety.

The government is currently considering the step in order to cut the number of children affected by spina bifida and other devastating "neural tube defects".

We need to be cautious before introducing such a policy

Professors Brian Wharton and Ian Booth

They want large scale trials to take place - just as if the vitamin was a new drug being tested before widespread use.

Currently only a third of women who become pregnant have been taking folic acid supplements at conception, even though it is known this can reduce the risk.

As the key period during which the condition is prevented occurs very early in pregnancy, it is too late to take it once pregnancy is detected - so women who become pregnant unexpectedly cannot be protected except by introducing folate into the national diet.

However, Professors Brian Wharton and Ian Booth, from the Institutes of Child Health in London and Birmingham, say the health effects of folate have not yet been researched thoroughly enough.

They wrote: "We need to be cautious before introducing such a policy.

"For example, in people with vitamin B12 deficiency, consumption of folate may mask megaloblastic anaemia - which may then progress undetected to subacute degeneration of the spinal cord.

"Folate also interferes with some antiepileptic or antifolate drugs."

Babies are continuing to be born with a severe, preventable impairment

Andrew Russell, Spina Bifida Association

They added: "We should learn the lesson of fortifying infant foods with vitamin D to prevent rickets - it did so, but only at the expense of hypercalcaemia in a significant minority."

However, Andrew Russell, the executive director of the Spina Bifida Association, which strongly backs the introduction of fortified foods, dismissed the worries of the two scientists.

He said: "The reservations raised by Professors Wharton and Booth were dismissed by
a government expert committee two years ago."

He accused the government of dragging its feet since the committee backed fortification.

"This is indecision, rather than prudence: untold suffering has been caused as hundreds of women choose to terminate affected pregnancies each year,
and babies are continuing to be born with a severe, preventable impairment."

In 1998 there were just under 400 pregnancies found to be affected by a neural tube defect.

Increasing the folic acid intake of the general population would prevent between a third and two-thirds of these.

A spokesman for the Department of Health said the delay had been unavoidable: "The complexity of the analysis and the need for additional work, particularly on the potential adverse effects of high folic acid intakes, led to a longer timescale than anticipated."